The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Internal combustion engine operation involves combustion that generates exhaust gas. During combustion, air is delivered through an intake valve and fuel is delivered through a fuel injector and mixes in the cylinder. The mixture is combusted therein. Air flow delivered to these cylinders can be measured using a mass air flow (MAF) sensor. The MAF sensor measures the total intake of fresh air flow through the air induction system, which may include one or more turbochargers. After combustion, the piston forces exhaust gas in these cylinders into an exhaust system. The exhaust gas may contain various emission components, including unburned hydrocarbons and particulates or soot.
Engine systems often include an exhaust gas recirculation (EGR) system to reduce engine emissions. EGR involves re-circulating exhaust gases back into the cylinders, which reduces the amount of oxygen available for combustion and lowers cylinder temperatures. An EGR system can enable ignition timing to remain at an optimum point, which improves fuel economy and/or performance. However, fouling of one or more components of the EGR system can occur if the temperature of the exhaust gas drops below a critical level. In particular, heavy hydrocarbons in the exhaust flow can condense and the soot particles therein can conglomerate and stick to the surface of the components.
The exhaust recirculation gas mixes with incoming air supplied to the intake manifold. The exhaust recirculation gas can thereby increase the temperature of the air flowing into the intake manifold. As the temperature of the air flowing into the intake manifold increases, an increase in the pressure of the flow is required to achieve the same mass flow rate of air to the intake manifold. As a result, the higher temperature can result in pumping losses and require the turbocharger to work harder. In extreme cases, if the pressure exceeds the capabilities of the turbocharger, a desired quantity of exhaust recirculation gas flow may not be possible thereby reducing the benefits to the emissions of the EGR system.
Typically, a single EGR cooler is utilized to meet the cooling requirements of the EGR system. Currently the EGR cooler is designed to meet the maximum EGR cooling required by an engine, usually at the highest EGR flow and high exhaust temperature. As a result, when the engine operates at lower EGR flow and/or lower exhaust temperature, the EGR cooler capacity exceeds the required level. This can cause the cooler exit temperature to drop below the critical temperature, thereby causing EGR cooler fouling. In an attempt to compensate for this, some EGR coolers have a bypass wherein the exhaust recirculation gas bypasses the cooler and, as a result, does not have its temperature reduced. When using a bypass, the exhaust recirculation gas may be at an undesirably high temperature. Thus, during some operating conditions the typical EGR system currently utilized either provides potentially overcompensation for the cooling of the exhaust recirculation gas or no cooling.